Improvement of Fatigue Strength and Toughnening of Particle Reinforced Composites

研究了颗粒增强长径比和力学性能对原位颗粒增强复合材料延性、静强度和疲劳强度的影响。采用金属材料延性断裂的力学模型和复合材料的矩形颗粒单元模型进行了定量分析。试验材料为原位真空电弧重熔法制备的TiC和TiB颗粒增强钛基复合材料和Ti-6Al-4V合金。TiC颗粒为球形，TiB颗粒为晶须状颗粒。采用光滑圆棒拉伸试件和缺口圆棒拉伸试件，研究了应力三轴度对塑性断裂过程和延性的影响。在变形早期观察到的颗粒开裂是微孔洞的第一个形核位置，导致塑性显著下降。无论是否存在缺口，两种材料均未观察到颗粒与基体界面的脱粘现象。力学模型和有限元计算假定陶瓷颗粒在基体塑性屈服时开裂。TiC/Ti复合材料的疲劳裂纹扩展速率高于Ti-6Al-4V合金，而TiB/Ti复合材料由于颗粒桥联和裂纹偏转而表现出较高的抗疲劳裂纹扩展能力。然而，裂纹形核控制疲劳寿命的光滑状态下的疲劳强度因颗粒开裂而严重恶化。界面的高强度会导致钛基复合材料疲劳强度的急剧下降。循环载荷作用于钛基材，导致钛基材的循环软化，导致颗粒开裂。研究结果表明，钛基复合材料的疲劳强度不可能高于基体，而铝基复合材料的疲劳强度往往高于铝合金。

Effects of aspect ratio and machanical properties of particle reinforcement on ductility, static and fatigue strength of in-situ type of particle reinforced composites were invetigated. Mechanical models for ductile fracture for metallic materials and the rectangular particle element model for composites were applied for quantitative analysis. Materials tested were titanium matrix composites reinforced with TiC and TiB particles produced by the in-situ vacuum arc remelting process, andTi-6A1-4V alloy. TiC particles are spherical and TiB are whiskerlike particles. The effect of stress triaxiality on ductile fracture process and ductility were invetigated using smooth and notched round bar tensile specimens. Particle cracking which is observed at early stage of deformation is the first nucleation site of micro-void, resulting significant degaradation in ductility. Debonding at interface between particle and matrix wasn't observed in both materials regardless of the presence of notch. Mechanical models and finite element calculation presume the cracking of ceramic particles at the plastic yielding of matrix. Strengthening of interface doesn't improve the ductility.Fatigue crack growth rate in the TiC/Ti composite is higher than that of Ti-6A1-4V alloy, whereas the TiB/Ti composite shows high resistance to fatigue crack growth due to particle bridging and deflection of crack. However, fatigue strength in smooth condition in which crack nucleation process governs fatigue life, is highly deteriorated by particle cracking. High strength of interface causes high degradation in fatigue strength of titanium matrix composites. Cyclic loading to the titanium matrix results cyclic softening of matrix and it causes particle cracking. The results in the present work indicate that higher fatigue strength than the matrix can not be expected in the titanium matrix composites, while the alminium matrix composites often show higher fatigue strength than that of alminium alloys.